I would let them soak for several hours before boiling to make sure they have water in every porous hard-to-reach area. That way when you boil you can be sure the water in those spots will boil as well. You never know what kind of bacteria is hanging out in the ground. Especially areas that have been sitting undisturbed for a few years - they can harbor anything from scarlet fever germs to natural anthrax (highly unlikely, but still, better safe than accidentally killing fish). I have heard of kids getting really horrible illnesses from eating sand and dirt that have been sitting around untouched.

Also, boiling may not be hot enough to kill everything. I would put the rocks on a cookie sheet and bake them at 450 degrees for at least 20 mins too. Obviously, don't touch them for a long time after this!

When you're done sanitizing, go over the rocks with a harsh brush, like a finger nail brush or something similar. If you're not getting any grit off the rocks at that point, they should be good to add to the tank.

+1 on the oven idea. This is what I did for all my lace rock in my cichlid tank:

1. ran them under the hose outside to get most of the big junk off
2. threw them (only a couple would fit at a time so it took forever) in the dishwasher with no soap (I also did a "pre-wash" with nothing in the dishwasher to make sure soap residue was mostly gone).
3. Took them out right when the last rinse cycle was done and threw them in the oven for ~25-30 minutes (again, a longer time since the lace rock was large and had lots of nooks and crannies) and then left them to cool in my dining room.